Hitchcocks Forum

I'm shortly going to be starting work on a 500 bullet classic project and have been thinking about work I'll need to have done in advance. This is a longwinded post as much for me to organise my thoughts as anything but comments or suggestions would be welcome



It's going to be stripped right down to the crankcases and I'll be having a bit of work done on it. I have the services of a proper, oldschool motor engineer locally so I'm going to get him to fit the mains and relieve the casting for fitting of high lift cams. I'm also going to see if I can get him to mill two of the stud holes to take a dowel for crankcase alignment.



Now my late model 2007 350 is one that breathes through the oil tank and has a return circuit to the timing chest from a catch can as standard. That has proven to be considerably less than ideal both in terms of breathing and mayonaise formation and after a lot of experimentation (I actually measured the gas flow output at idle with various brearther arrangements using a bucket of water and a wine bottle) I finally fitted a 1/4" nipple in the crankcase itself for a breather which vastly reduced oil consumption and improved performance.



The donor engine for this project is a 2003 bullet and it is kind of halfway. It has the oiltank breather going to a catch can but no timing chest return (presumably you're supposed to empty the can when it's full or simply leave it to overflow?). The standard breather spigot is present on the casting just below the barrel but hasn't been drilled and is pretty rough. The corresponding "slot" on the inside of the casting is present.



It makes sense to me to reinstate the standard breather system while I'm this far in the motor. It worked for 50 years or more and I don't see why I want piston blowpast vapour in my oil tank. What I'm thinking of doing is closing up the hole between the crankcase and oil tank with durafix solder and open up the standard breather for fitment of a duckbill or other non-return valve. I think it would look and work more neatly if the original (very rough and slightly tapered) cast stub was removed altogether, a hole drilled through into the slot in the crankcase and a flat area machined onto the outside so a triumph type alloy breather stub can be fitted.



On my 350 I've actually attached the breather to a piece of 1/4" copper pipe that comes out of the crankcase then runs vertically behind the barrel before attaching to a flexible hose with a non-return valve in that eventually exits the bike off the back mudguard. I find this breathes very freely. The rationale of the vertical copper pipe is it should minimise the amount of oil that escapes and at the same time the heat from the engine should encourage water vapour to remain as such rather than forming mayo and choking the breather system.



One other thing I was considering doing was enlarging the (surprisingly small) return hole between the timing chest and oil tank slightly. Perhaps by filing it into an oval shape so the area of the hole is increased but the oil level in the timing chest remains the same.



A small filter on the oil tank breather stub would ensure it doesn't form a vacuum in the oil tank as it empties.

Gday mate.
During a recent crank change, I enlarged the internal breathing hole between the crankcase and oil tank, and drilled and tapped the crankcase/barrel 'stub' for a 3/8 hose fitting.
The engine now has two 3/8 breather pipes into a modified filter equipped catch can. (with the standard return to the timing chest). This cured the issues I had. Regards Paul.

*Edit- modified AIR filter equipped catch can

Does yours breathe out through both breathers?



My 350 breathes OUT through the crankcase one and IN through the oil tank one. With the oil tank sealed up and the crankcase breather open, it blows 1.8litres/minute out at idle. With both open it breathes out 10litres/minute from the crankcase breather and has a vacuum (not measured but it will suck the hose onto your finger tip) at the oil tank one.



I suppose this confirms the crankcase breather is generating the desired vacuum. I opted for leaving both open with the crankcase venting to atmosphere and the oil tank breather attached to a small air filter in the toolbox to stop it sucking in nasties and water.

I should add that the fluid that comes out of the end of the breather pipe is nasty. A brown, watery emulsion. It's not something I'd pour into my engine.



It has also re-instated the trademark small drip of oil people expect to see where a Brit bike has been parked, something my extensive use of bonded seal washers had abolished.

This engine (2003 Bullet) breathes through both 3/8ID lines, via the catch can. The can itself now has separate ports for each feed, a free air high flow filter, and then the factory return line to the timing chest. No one-way valves anywhere in the system. The original 'comunication' drilling between crankcase and oil tank now acts as 'balance' port. It was enlarged from it's original 3/16 size to 1/2inch. These mods came from the same principals applied to engine breathing for our drag(car) engines.

The engine breather requirements for a single cylinder low reving is different to a multi-cylinder high reving race motor. That which is good for your drag-car is definitely not optimal for the Bullet.


Do not think along the lines that the engine has to displace 500cc (?) of air on every stroke; it does not. For the typical street motor, all that is desired from a breather to to achieve a net negative crankcase pressure so as to augment ring sealing on the bore. Typically it is not until a single cylinder (vertical twin) is reving past 5000rpm that large bore, long pipe, open breathers become effective. AS your initial experiments showed, a small vent is all that was needed.


Installing a breather off the exhaust rocker cover has merit.

Presume you mean there will never be a vacuum in an oil tank that doesn't connect to the crankcase? Because later bullets breathe only through the oil tank and are presumably intended to have the usual slight vacuum?



I've seen pictures of bikes with breathers in the rocker covers before but wasn't sure what they were intended to achieve. Which way are they breathing (in or out)? I'd have expected them to be at roughly neutral pressure under normal running or does the oil pumping pressurise them more quickly than it can escape down into the timing chest and back into the oil tank? Do they fit non return valves on those?



It's one of those arcane bits of motor engineering I find I want to get my head round, a bit like how the hell 2-strokes manage to suck and blow at the same time and how mid 80's kawasaki sportsbikes manage to suck 110% of their engine capacity through the airbox on each cycle.

In the post 2003 Bullets with the breather off the roof of the oil tank, their is an excess of fume being blasted around inside the engine. What once was directed out of the crankcase, now jets across the surface of the oil in the tank. When the Oil tank is over filled it is often the case that the oil is whipped up and blown out of the breather. The same fume is also able to pass into the timing chest and pressurise the Rocker covers. You will also note that the retaining nut for the tappet cover has an O ring seal in its face to stop oil from being blown out.


All in all this later breather was a poorly thought out mismash to meet emissions standards and should have had more thought applied in the design stage...


Some have advocated the need to run a lower oil level to minimise the Oil mist carry-over effect. I do not, rather I simply make sure the the outlet pipe maintains a constant steady rise to permit oil droplets to fall out and return freely to the oil tank, whilst allowing the lighter volatile fractions and steam to be expelled


There is also a degree of combustion byproducts passing between the exhaust valve stem and its guide, into the engine. Hence the advantage of venting the ex rocker via a Duck-bill hose


(The early AVL motors have a breather off the head)


While you have your cases apart, consider fitting a wire-mesh diffuser over the oil tank breather outlet inside the tank, so as to restrict the possible free passage of oil mist up and out of the breather pipe...




Why is an engine able to achieve 110% volumetric efficiency? Mass. At higher rpm the mass of air in motion keeps flowing into the engine long after the piston has passed BDC. The overall effect in augmented by inlet and exhaust port design and maintaining optimal air velocity in conjunction with Valve closing points. There is a big slug of rapidly moving air entering the engine and its kinetic energy effectively supercharges itself.


Long gentle taper inlet tracts combined with megaphone exhaust, are designed to push/pull gases into and out of the engine.

Thanks for the reply. Lots to think about.



How do you go about putting a breather in the rocker cover? A simple threaded nipple? A banjo and bolt? Or I dimly recall seeing a couple of pictures of fairly heavily tuned bikes with what appears to be -after a degree of searching- part number 41540 "breather body" bolted on in odd positions?



I wonder if there's enough meat on the cases to machine a large enough flat to fit one of those where the conventional cast crankcase breather stub is.